Houston...we have a problem....any advice or help?

[Bigprairie1]

Ok, after many, many faithful seasons my 20+ year old force (1990/90hp/3 cylinder) may have pooped it's pants.

I was about 100' from the dock, throttled it back and noticed the motor running rough and poor...but still running. After another 15' it stalled out...then restarted and limped back home. I pulled the plugs today and did a compression test and it reads as follows: Top cylinder: 115 psi, centre cylinder 65-70psi and the bottom cylinder 120 psi.
So.....did I break a ring?...or..two rings or a reed valve or????
I intend on pulling the head off and taking a look in there and will post pictures...but any feedback, advice or history that you may have fellow IBoaters with this predicament and how to fix it would be greatly appreciated.
Now...on the upside, I bought another complete 1991 Force 90hp several years ago which has been gathering dust in the basement. It has 140+psi compression all cylinders but I need to install the flywheel on it and the leg to get it 'back on the water'.
All input is most welcome.
Thanks guys!!
BP

 

[jerryjerry05]

Re: Houston...we have a problem....any advice or help?

Might be as simple as a bad head gasket.Or a rebuild.
After the inspection and you find anything BIG then swap to the back up.
Can be done in a day if you have a lift.
Removing the flywheel after 20 yrs can be tricky.See pic that's how I do it.
Loosen the nut don't remove.Install BIG 3 jaw puller.Tighten with an impact untill it's really tight.Then a hammer on the head of the screw.Should pop off.Might need to tighten a bit more if that doesn't work the first time.

 

[bentle]

Re: Houston...we have a problem....any advice or help?

Make sure you don't hit so hard that you cause even bigger prob.like crank shaft
bearing damage or seal damage. Minimum force required. Spray a little pb-blaster on the top of shaft after you get the nut off and let it sit over night really helps.
Safety glasses are very important with this little project as with any other repairs.

 

[Bigprairie1]

Re: Houston...we have a problem....any advice or help?

I didn't consider the issue of it being a head gasket....hmmm. To my knowledge the motor has never been rebuilt...but it didn't have that many hours on it. Up until the other day I'm guessing probably around 400 hrs since new or thereabouts...but it did have 21 years of age on it.
I was wondering that if it is a broken ring...is it possible to just change out that cylinder? (I think I read a posting about carefully working from the intake side of the motor to do this??).
Would a broken reed valve lead to low compression? Or does that typically go unnoticed until it fries the whole motor from a lean condition?
As for the other/spare 90hp....fortunately I made a top-jacking plate to get the original flywheel off. The flywheel that was on it had the magnets break so hence the need for a new flywheel on that motor.
I'm wondering what is less work.....assess and possibly redo the existing '90/90hp motor....or remove it and install/rig the other '91/90hp..which needs a little bit of re-assembly before installation. The motors are pretty much identical except for the fact that 1990 motor runs the light blue/Prestolite ignition and the 1991 motor runs the next series Merc/based ignition.
Again, all feedback and comments on this and assessing this situation are completely welcome and needed.
Many Thanks!!
BP